Communicate lack of support for feature-state in layout properties

[samanpwbb]

When reading up on feature-state, I noticed this line:

mapbox-gl-js/src/style-spec/validate/validate_expression.js

Line 24 in 872c372

return [new ValidationError(options.key, options.value, '"feature-state" data expressions are not supported with layout properties.')];

– does this mean that layout properties like icon-image and text-size cannot be used with the feature-state expression? If that is the case, could we capture this constraint in the spec documentation:

mapbox-gl-js/src/style-spec/reference/v8.json

Line 2609 in 1a8e9fe

"doc": "Retrieves a property value from the current feature's state. Returns null if the requested property is not present on the feature's state object.",

?

Right now, there's no way to know that we're supposed to hide this expression in the Studio UI for layout properties with data driven styling support.

[asheemmamoowala]

@samanpwbb Are you thinking that a note in the doc field would be enough, or are you looking for an additional field in the style-spec to automate the Studio UI?

[samanpwbb]

An additional field would be best.

[jfirebaugh]

How about adding "feature-state" as a potential element of the "parameters" array property added in #6521?